Corrosion problems in petroleum refining operations associated with naphthenic acid constituents in crude oils have been recognized for many years. Such corrosion is particularly severe in atmospheric and vacuum distillation units at temperatures between 400 degrees F. and 790 degrees F. Other factors that contribute to the corrosivity of crudes containing naphthenic acids include the amount of naphthenic acid present, the concentration of sulfur compounds, the velocity and turbulence of the flow stream in the units, and the location in the unit (e.g., liquid vapor interface).
In the distillation refining of crude oils, the crude oil is passed successively through a furnace, and one or more fractionators such as an atmospheric tower and a vacuum tower. In most operations, naphthenic acid corrosion is not a problem at temperatures below about 400 degrees F. Traditional nitrogen-based filming corrosion inhibitors are not effective at these high temperatures and the other approaches for preventing naphthenic acid/sulfur corrosion such as neutralization present operational problems or are not effective.
It should be observed that the term "naphthenic acid" includes mono and di basic carboxylic acids and generally constitutes about 50 percent by weight of the total acidic components in crude oil. Naphthenic acids may be represented by the following formula: ##STR1## where R is an alkyl or cycloalkyl and n ranges generally from 2 to 10.
Many variations of this structure and molecular weight are possible. Some practitioners include alkyl organic acids within the class of naphthenic acids.
Naphthenic acids are corrosive between the range of about 210 degrees C. (400 degrees F.) to 420 degrees C. (790 degrees F.). At the higher temperatures the naphthenic acids are in the vapor phase and at the lower temperatures the corrosion rate is not serious. The corrosivity of naphthenic acids appears to be exceptionally serious in the Presence of sulfide compounds, such as hydrogen sulfur.
Efforts to minimize or prevent the naphthenic acid/sulfur corrosion have included the following approaches:
(a) blending of higher naphthenic acid content oil with oil low in naphthenic acids; PA0 (b) neutralization and removal of naphthenic acids from the oil; and PA0 (c) use of corrosion inhibitors.
Because these approaches have not been entirely satisfactory, the accepted approach in the industry is to construct the distillation unit, or the portions exposed to naphthenic acid/sulfur corrosion, with resistant metals such as high quality stainless steel or alloys containing higher amounts of chromium and molybdenum. However, in units not so constructed there is a need to provide inhibition treatment against this type of corrosion. The prior art corrosion inhibitors for naphthenic acid environments include nitrogen based filming corrosion inhibitors. However, these corrosion inhibitors are relatively ineffective in the high temperature environment of naphthenic acid oils.